Strain, ε, is a dimensionless ratio that indicates the deformation of an object subjected to a stress, for example from an external force or due to expansion or contraction based on a change in temperature or other physical characteristic. Strain can occur in any direction of a coordinate system. Uniaxial strain is strain resulting from a stress exerted in one direction only (that is, along only one axis of) a coordinate system. FIGS. 1A and 1B illustrate a case of uniaxial strain on a rod of initial length, L, which is stretched to a length, L1. As shown in FIG. 1A, the strain is defined as the change in length, L1−L, with respect to the original length, L, orε=(L1−L)/L 
The initial length, L, is referred to as the “gage length,” the strain sensitive length of the strain gage, and in this example the strain is the change in length over the gage length.
As shown in FIG. 1B, strain at a point is defined by considering an arbitrary point, P, which has a position vector, x, and its infinitesimal neighbor, dx. Point P shifts to P1, which has a position vector x1 after application of a load; meantime, the small increment of change in length is dx1−dx, and strain at a point isε=(dx1−dx)/dx=du/dx 
Thus, two points defining a straight line on the surface of a body can be chosen to define a gage length. If the distance between the points changes over time relative to the gage length due to deformation of the surface, the average strain over the gage can be determined. Strain is shown in FIG. 1 relative a tensile force but the development is equally valid for a compressive force.
Technical efforts by Vachon and Ranson (see U.S. Pat. No. 4591,996 and U.S. application Ser. No. 10/890,994) have continued in the area of optical correlation of surface images to detect strain. Specifically, these efforts include, among other things: (1) optical detection of edges of images on surfaces as well as optical detection of edges of surfaces, (2) optical correlation of dot and other geometric patterns applied to surfaces, and (3) optical correlation of the movement of centroids of geometric patterns applied to surfaces. All of these analytical and experimental efforts have been directed to optical detection of strain.
Single-element electrical strain gages and electrical resistance gages arranged in a rosette pattern employ analog techniques, rather than measuring strain directly. Previous optical correlation techniques calculate strains using a convolution integral, and also do not measure strain directly.
It is to the provision of a strain gage that can measure strain directly, as well as assessing fatigue damage, that the present invention is directed.